1. Field of the Invention
The present invention relates to a method of manufacturing noble metal electric discharge chips adapted for use in long life spark plugs, and to a method for manufacturing the spark plugs.
2. Description of the Related Art
In order to secure the spark wear resistance required of a spark plug, an iridium alloy having a high melting point has been employed as a material for noble metal electric discharge chips. However, an iridium alloy containing iridium as a main component and containing metals other than iridium in an amount of not higher than 35% by mass is very difficult to plastically deform. There is a method (for example, JP-A-10-32076, referred to herein as Patent Document 1; and JP-A-200-33170, referred to herein as Patent Document 2) of forming noble metal electric discharge chips having a small cross-sectional area by subjecting an ingot of such an alloy containing iridium as a main component to hot processing.
However, when an iridium alloy is subjected to hot processing as disclosed in Patent Documents 1 and 2, the occurrence of cracks, and the disconnection and folding over of portions during the wire drawing operation, cannot be sufficiently prevented. As a result, it is difficult to adequately improve the manufacturing yield of discharge chips.